DRUG CALCULATIONS - Christchurch Hospital

DRUG CALCULATIONS

When calculating how much of a drug is required, working with the formula helps the accuracy of the calculation.

Always remember this formula:

What you want X Quantity it comes in

What you have

1

It's OK to use a calculator!

1. A patient requires 4 mg of Morphine IVI. Morphine is available as 10mg/ml. How many mls will you draw up?

2. Gentamycin 360 mg is prescribed. Gentamycin is available as 80mg/2ml. How many mls will you draw up?

3. Haloperidol 3 mg IVI is charted. Haloperidol is available as 5mg/ml. How many mls is required?

4. Frusemide 70mg IVI is charted. Stock dose is 20mg/ml. How many ml would you give?

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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.

METRIC CONVERSIONS

Metric weights and measurements involve a step-by-step conversion from one unit to another.

With weight we often convert to smaller (and more numerous units) thus:

Kg

gm

mg

mcg

grams milligrams micrograms

Each of these steps involves the heavier unit being multiplied by 1000 to bring up the number of the smaller units for the same weight.

kg

g 1kg x 1000 = 1000g

g

mg 1g x 1000 = 1000mg

mg

mcg 1mg x 1000 = 1000mcg

When we multiply by 1000 we move the "decimal point" three places to the right

0.5g == ?? mg

0.5 x 1000 == 500mg 1 place = 5 2 places = 50 3 places = 500

If we are converting from a lighter unit to a heavier unit we move the decimal point three places to the left for each conversion. Another way of putting it is we divide by 1000.

mcg

mg 1000mcg 1000 = 1mg

mg

gm 1000mg 1000 = 1gm

gm

kg 1000gm 1000 = 1kg

500mg = ?? gm

500 1000 = 0.5mg 1 place = 50 2 places = 5 3 places = 0.5

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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.

Try these:

5. Atropine 0.6 mg = ?mcg

0.6 x 1000 = 600mcg

6. 0.01gm = ? mg

0.01 x 1000 = 10 mg

7. Gentamycin 360mg = ?gm

360 1000 = 0.36gm

8. Digoxin 125mcg = ? mg

125 1000 = 0.125mg

If we are moving across two conversions we repeat the process twice. For example from grams to micrograms:

3gm converted to micrograms Step one:

= 3 x 1000 = 3000mg Step two:

3000mg x 1000 = 3,000,000mcg

9. Augmentin 1.2gm = ?mcg

1.2 x 1000 = 1200mg 1200 x 1000 = 1,200,000mcg

10. Digoxin 125mcg = ? gm

125 1000 = 0.125mg 0.125 1000 = 0.000125gm

Memory Tip: "Heavy to light ? three places to the right"

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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.

Try these: Convert 11. 1.2 mg to mcg 12. 1.3g to mg 13. 500mcg to mg 14. 0.04 mg to mcg 15. 20mcg to mg 16. 600mcg to g 17. 2g to mcg

When we are converting volumes the process is the same.

Litres 1 Litre

0.25L 375ml

ml 1000ml

250ml 0.375L

Try these: Convert 18. 450ml to L 19. 64 ml to L 20. 4.3L to ml

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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.

MG/KG DOSE CONVERSION Often we need to check the dose of a drug or solution that is based on the patient's weight. We multiply the prescribed mg dose by the kg weight of the patient

Prescribed dose - 25mg per kg Patient's weight - 66kg What is the dose required?

25mg x 66kg = 1650mg = 1.65g

21. Prescribed dose ? 50 mg/kg Patient's weight ? 79 kg What is the dose required?

22. The patient is charted 15mg/kg/day. The patient weighs 75kg. a) How much is the total dose per 24 hours?

b) How much will the patient receive every 8 hours?

ROUNDING OFF the decimal point Rounding up if greater than 5, round up Rounding down if less than 5, round down

eg 166.66 = 167 eg 33. 33 = 33

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C:\Documents and Settings\neilh\Local Settings\Temporary Internet Files\Content.Outlook\BA80LT24\Clinical calcs REVIEW SESSION.doc Prepared by: Janet Tweedy and Deb Mason, Nurse Educators, Department of Nursing, Christchurch Hospital, May 2000. Canterbury Health Ltd. Last updated June 2000.

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